PTL 1 discloses a boosting process (a process in which primary electron beam is caused to pass in a SEM column with high accelerating voltage with a sample kept at ground potential and to be decelerated to ground potential immediately before passing an objective lens). A boosting process can realize, with low accelerating voltage, resolution much higher than a case where a single out-lens objective lens in which magnetic field is not immersed on a sample is used. Note that, in a commercial SEM with a boosting process, a schottky electron source (SE electron source) is mainly used.
PTLs 2 and 3 disclose a cold cathode (CFE: cold field-emission) electron source in which the electron source is not always heated. The current stability (10%/min) of the CFE electron source is extremely worse than that of the SE electron source. A SEM with a CFE electron source generally performs flushing to instantaneously heat the electron source in order to clean the surface of the electron source. Note that, emission current of the electron beam is reduced or varied by absorption and desorption of residual gas around the electron source after flushing. The longer the time required for the emission current to be reduced is, the more current stability of the CFE electron source is improved. Since decrease of emission current depends on the amount of residual gas around the electron source, a technique to further improve current stability by reducing residual gas around the electron source as much as possible with a non-evaporable getter pump in an electron gun chamber including the electron source is disclosed (PTLs 2 and 3).
PTL 4 conceptually discloses a SEM with a combination of a CFE electron source and a boosting process.